Protonation Constants of Ethylenediamine, Diethylenetriamine, and Spermine in NaCl(aq), NaI(aq), (CH₃)₄NCl(aq)_, and (C₂H₅)₄NI(aq) at Different Ionic Strengths and <i>t</i> = 25 °C

Abstract

The protonation constants of three linear unsubstituted polyamines were studied potentiometrically at 25 °C in different ionic media and at different ionic strengths (NaCl, NaI, and (CH3)4NCl: I ≤ 2.5 mol·L-1; (C2H5)4NI: I ≤ 1 mol·L-1). The general trend for log KiH values is NaCl ≥ NaI ≫ (CH3)4NCl > (C2H5)4NI. The dependence of the protonation constants on ionic strength was taken into account using a simple Debye−Hückel type equation (molar concentration scale) and the SIT (Specific ion Interaction Theory) approach (molal concentration scale). Differences in protonation constants in the different supporting electrolytes were also interpreted in terms of complex formation by considering the weak species (A = amine; X- = Cl-, I-; M+ = (CH3)4N+, (C2H5)4N+) HiAX(i-1) and MHjA(j+1) with i = 1 ... m and j = 0 ... (m − 1), where m is the maximum protonation degree of the polyamine. The formation constants of these species are linearly dependent on the number of amino groups and on the protonation degree of the polyamine. Some empirical relationships are reported.